1. Field of the Invention
The field of this invention relates to techniques for well completions and more particularly to the use of plugs during underbalanced well completions.
2. Background of the Prior Art
One of the primary goals during drilling and completion operations of wells is to protect the producing formations from damaging effects associated with fluids lost into the formation during the drilling and completion operations. A secondary goal is to eliminate excessive losses of expensive drilling and completion fluids.
Most wells currently are completed conventionally using kill-weight fluids which might impair the formation. Even if a well is drilled underbalanced (formation pressure greater than wellbore pressure), the well typically is exposed to damaging fluids during completion operations. This process defeats the principle purpose of underbalanced drilling which is to avoid impairment of the formation.
In situations using prior art apparatus and techniques, particularly those involving deviated wellbores, the initial portion of the well is drilled and a casing is set. The casing is then cemented. After the cement sets, the deviated portion of the wellbore is drilled. Prior designs have involved running a liner string into the wellbore after completion of the drilling of the deviation in the wellbore beyond the cemented casing. An inflatable packer has been inserted through the liner string to isolate the formation while the bottomhole assembly is assembled into the wellbore above an inflatable bridge plug. Certain problems, however, have developed in particular applications with the use of through-tubing inflatable bridge plugs. For one thing, the ability of the through-tubing inflatables to hold particular differentials can be problematic, especially if there are irregularities in the sealing surface where the plug is inflated. Additionally, due to the compact design required in certain applications, the through-tubing inflatable element cannot expand far enough to reliably hold the necessary differential pressures that might exist across the inflated bridge plug. Finally, there could also be difficulties in retrieval of the through-tubing inflatable bridge plug back through the string from which it was delivered.
The flexible nature of the through-tubing inflatable design could also create problems if it was decided simply not to retrieve the plug after putting together the bottomhole assembly above it. The slender design of the through-tubing inflatable plug could create advancement problems if the plug were to be merely pushed to the bottom of the hole with the production tubing. If any washouts in the deviated portion of the wellbore were encountered by the bottomhole assembly with the deflated through-tubing plug at the front, then the entire assembly might get stuck prior to its being advanced to the bottom of the wellbore for proper positioning. Generally, the through-tubing designs have not provided a circulation passage therethrough to facilitate advancement of a deflated plug into the uncased portion of a wellbore using circulation.
To prevent formation damage and fluid loss and to maximize productivity of a well, the well needs to be drilled and completed underbalanced. The method and apparatus of the present invention address many of the problems associated with conventional completion techniques by providing a downhole tool such as an inflatable bridge plug which can be set at the desired location to isolate a portion of the wellbore. The tool is securely positioned to enable it to withstand substantial differentials. After the tool is positioned, the bottomhole assembly can be put together in the wellbore above the tool where the wellbore is isolated from the producing formation. The invention accomplishes the objective of removing the plugging device from the path by deactivating it and moving it within the wellbore. By carrying the plug with it within the wellbore, the deactivating apparatus gets the benefit of additional structural rigidity which allows it to advance to the bottom of the hole with less chance of hangups in washouts.
Other advantages of the apparatus and method include a physical support (anchor) for the plug to facilitate its being enveloped after it is deactivated. The design also facilities flow of circulation fluid through the deactivation tool and encapsulated plug as it is being moved to the bottom of the hole.
Another embodiment of the present invention is an inflatable swab valve that is installed as a part of the casing. During operation, the swab valve is inflated inwardly to seal the well. The seal is deactivated for later operations and downhole apparatus pass through the cavity formerly occupied by the inflated membranes of the swab valve.